1. Field of the Invention
This invention relates to exercise apparatus. The invention is particularly applicable to exercise apparatus designed to simulate the motion of a travelling body.
2. Description of Related Art
Exercise apparatus is known which allows the user to simulate an exercise in the form of human-powered transport, or simply walking or running. Among these are treadmills, rowing machines and exercise cycles. They have been developed to allow the user to perform an exercise in a confined space that would otherwise require a large area. Other forms of exercise apparatus provide a force to exercise against. In this, they are static (producing a torque to exercise against) as opposed to dynamic (producing a motion).
One of the basic aspects of most types of apparatus of this kind is the simulation of the momentum of either the human body or the transport being simulated. This is commonly achieved by using a flywheel linked to the apparatus, counter to the inertia of which the user exerts a force in performing the exercise. As an example of this, the exercise treadmill provides a so-called `rolling road` in the form of a conveyor belt powered by an electric motor. Typical motors are induction motors, brushed permanent magnet motors and brushless dc motors.
The `runner` moves relative to the belt but actually remains substantially stationary. To take the weight of the runner, the flexible belt travels across a support such that the runner's leading foot hits the belt immediately above the support and is carried backwardly. The impact of the foot on the belt pinches the belt between the foot and the support creating a sudden load on the motor. The speed of the travelling belt is maintained by a flywheel operably mounted in relation to the motor so that little or no change in the speed of the belt is perceived by the runner as a result of the foot hitting the belt. Similarly, there are occasions in the running cycle when both feet are out of contact with the belt and it is equally important that the speed of the belt is not substantially increased before the next foot to land makes contact with the belt.
From this it will be appreciated that using a treadmill exercise apparatus involves the relatively sudden imposition and relief of loads on the motor as the feet perform the running action. Known drive systems which are cost-effective in such apparatus are unable to maintain the belt at a sufficiently constant speed. In order to reduce the speed fluctuation to an acceptable level, the flywheel is used to increase the inertia of the rotating components and damp out short-term fluctuations.
The mechanical dynamics of the system are dominated by the inertia of the flywheel and the friction in the belt/roller system. The system therefore has very slow and well-damped dynamics, and any electrical or mechanical disturbances will be substantially suppressed. Any device used for torque or speed-control feedback may accordingly be of relatively low quality, in order to maintain overall costs.
It is well known that flywheels, by their nature, are relatively heavy items and often of a size which makes them awkward to integrate into a housing for the other, significantly smaller, components that will be associated with powering a piece of exercise apparatus. The presence of the flywheel in an exercise apparatus of the type described may significantly increase the size of the unit overall.
If the flywheel is removed from a prior art exercise machine in an effort to save cost and weight, the source of mechanical inertia is essentially removed. Thus, the control system will demand rapidly changing amounts of torque from the motor as the runner's foot lands on the moveable surface. The motor typically employed in such a machine has a relatively low bandwidth. It is therefore unable to react quickly enough to the change in torque demand and the speed of the moveable surface accordingly varies to an unacceptable degree. Attempts to improve the response time by increasing the bandwidth of the controller tend to be counterproductive as the controller cost rises dramatically and the overall response time of the system is limited by the motor's bandwidth.
As a practical matter, the standard of flywheel that is cost effective to use in exercise apparatus may well be inadequately balanced. The motor typically runs at 5000 rpm, which can mean that an inadequately balanced rotating flywheel gives rise to objectionable vibration while the apparatus is in use.
A further disadvantage of the use of a flywheel in exercise apparatus is that it can take a considerable time for the exercise machine to come to rest when the power is removed from the drive motor. This can have undesirable consequences in the event of the user stumbling and operating an emergency stop.